A spokesperson said: "Offices were called to the northbound carriageway at Bucknalls Lane overbridge to reports that a man had fallen from the bridge. A number of motorists stopped to assist and subsequently one of these drivers was struck by a vehicle. Both men died at the scene.

"The road was closed for a number of hours whilst investigations took place at the scene. Both carriageways are now open.

"Officers are appealing to anyone who was in the area and saw what happened or has any information about events prior to the original incident to contact them."

Rupert Gadd from the Hertfordshire, Bedfordshire and Cambridgeshire Police Road Crash Investigation Unit told Heart Radio: "A male is know to have jumped from the bridge parapet onto the carriageway.

"Other motorists have stopped to assist to his aid - unfortunately one of those people who stopped was struck by another vehicle. We've ended up with two male fatalities.

"We have started an investigation into the collision and deaths of both males."

Rebecca Martin, who was travelling northbound on a Megabus told the Watford Observer, said: "We came to a screeching halt and it sounded like a car accident had happened in front of us or something. We then found out a man had jumped from the bridge in front of us.

"We were just three or four cars behind it. Then another car stopped and a man got out to help and was hit by another car. An air ambulance came and took both of the men away."

She added: "It was quite worrying when we came to a screeching halt and it was quite close to being a massive accident. It's really bad it's happened and that someone that was trying to help got run over is just awful."